A user equipment (UE) accessing IM core network (CN) subsystem services requires an IP address that is part of an IP addressing domain of a visited network IM CN subsystem. This is established using an appropriate PDP (Packet Data Protocol) context. For routing efficiency this context should be connected through a GGSN (General Packet Radio Services Gateway Support Node) in the visited network.
Every IM CN subsystem subscriber has a private identity assigned by the home network operator and used e.g. for registration, authorization, administration, and accounting purposes. This private identity is a unique global identity valid for the duration of the user's subscription with the home network and takes the form of a Network Access Identifier (NAI) as defined in the IETF (Internet Engineering Task Force) specification RFC 2486. The NAI for the private identity may contain the IMSI (International Mobile Subscriber Identity). The private identity is securely stored in the USIM (Universal Mobile Telecommunications System Subscriber Identity Module) and cannot be modified by the UE.
Furthermore, every IM CN subsystem subscriber has one or more public identities assigned by the home network operator and used by any user for requesting communications to other users. For example, these public identities may be indicated on a business card. Both telecom numbering and Internet naming schemes can be used to address users depending on the public identities allocated to the users. The public identities may take the form of a SIP URL (Session Initiation Protocol Uniform Resource Locator), as defined in the IETF specifications RFC 2543 and RFC 2396, or an E.164 number. At least one public identity is stored in the USIM.
In SIP as described e.g. in the IETF specification RFC 2543, each public identity is registered separately, as indicated in FIG. 1. Therefore, the registration of other subscriber identities requires additional registration procedures if a subscriber wishes to use multiple identities.
The subscriber may have several profiles that he would like to use separately e.g. business profile, private profile, chess club member profile etc. In the case “one private identity but several user profiles”, as shown in FIG. 2, the public identities that belong to the same user profile are bundled together and therefore they are also administrated together. In the case “several private identities”, as shown in FIG. 3, every user profile has its own private identity.
The subscriber registers with one of his public identities and lists all the other public identities that he wants to be registered with at the same time. When a subscriber has registered with one of his identities, he can use his other identities for outgoing calls without any further registration but he doesn't receive any calls with those identities. The subscriber can explicitly change an existing registration or make a new registration with whichever of his identities for incoming calls only, for outgoing calls only, or for both incoming and outgoing calls. He can make one or several registrations at a time.
Furthermore, the subscriber can register with one of his public identities specially indicating that all public identities should be registered, and all his public identities will be registered. As an option the subscriber can register without the mentioned indication, and all his public identities are registered by a default setting.
The subscriber may have several public identities. Normally, he registers with one public identity. If he wants to use another of his public identities e.g. to make a business call in the evening from home, he has to register with that identity before he can use it. It would be desirable to be able to register with all or with some of the public identities all together at the same time. After the registration, that public identity is valid also for incoming calls e.g. people may call home in business matters.